This invention relates to precision fasteners, and more particularly to precision fastener assemblies that incorporate a spring.
Precision fasteners are used to couple components together with a measured or otherwise approximately known amount of force. One class of precision fastener assemblies accomplishes this purpose by incorporating a spring having a known ratio of force to compression distance. For example, a given spring might be capable of applying X pounds of force in the axial direction for every Y inches of compression relative to a relaxed length. The ratio of force to compression for such a spring could be specified as X:Y. Fastener assemblies that operate according to this principle generally include a threaded fastener, such as a screw, having a known thread pitch. Because the thread pitch is known, a precise degree of axial displacement can be achieved with a corresponding number or turns of the screw. The screw and spring are oriented so that the spring is compressed as the screw is turned. Thus, a precise amount of spring force may be applied by turning the screw a measured number of times.
One problem associated with such fastener assemblies is the relationship between precision, axial displacement and force magnitude: Generally, high precision force applications are more achievable using springs that have a relatively low force to compression ratio. In other words, the greater the ratio of force to compression distance for a spring, the more difficult it is to apply a precise amount of force using the spring. On the other hand, springs having a high ratio of force to compression distance are advantageous to use because fewer turns of the threaded fastener are required for such springs to achieve large-magnitude forces. Fasteners that require only a few turns during their application help to make manufacturing easier and less time consuming.
It would therefore be desirable to have a precision fastener assembly that is capable of applying large-magnitude forces in a precise manner and that does not require a large number of turns during its application.
In one aspect, an assembly according to the invention includes a threaded fastener having an axial shaft. A spring is disposed coaxially around the shaft and is capable of compression and expansion in the longitudinal direction defined by the shaft. First and second stops are disposed along the shaft. The spring is pre-compressed and is disposed between the stops so that its expansion is limited by the stops. One end of the spring is mechanically coupled to a work piece. When the fastener is turned, relative displacement develops between the shaft and the work piece. The displacement causes the spring to further compress as the work piece end of the spring and the second stop move away from one another. The force of the spring is transferred to the work piece.
Because the spring is pre-compressed prior to application of the fastener to the work piece, large-magnitude forces may be achieved with relatively few turns of the fastener at the time of its application. In addition, relatively low force to compression ratio springs may be used to implement the device. Thus, the inventive fastener not only is capable of maintaining the precision associated with low-ratio springs, but is also simultaneously capable of achieving large-magnitude forces while requiring relatively few turns of the fastener at the time of its application to the work piece.